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So I'm almost there with restoring a late 60's Vox AC-30 (JMI circuit). Some questions I'd like to seek the group's thoughts on. Many thanks in advance!
1) In attempts to make as period correct as possible, will look to change back all metal/carbon film resistors to carbon comp resistors. It appears the market supply of 1 watt carbon comps is severely depleted everywhere. May I confirm using 1/2 watts should be fine? I'm inclined to focus more on the tone signal path and perhaps leave metal film on the power side (and even the vib/trem circuit as I don't really use all that much)
2) I noticed the currently installed dual main filter capacitor "can" (C39, C40) is labelled/rated at 32uf/32uf rather than the expected 16uf/16uf as listed in the schematics. I'm tempted to switch this back, but I feel I should perhaps at least investigate possible reasons a previous tech may have installed the higher capacitance. I'm seeing mixed comments with regards to Vox possibly intentionally upping the value in the late 60's. Anything I should particularly measure or take a look at to avoid frying something by switching back to 16uf? Any difference in sound I should expect? Bear in mind this used to be modded and wired for solid state rectification, and I reverted back to GZ34 tube rectification. I see a lot of feedback that 16uf is way too small for "today's standards" but I'm not sure what that really means? Safety standards, or tolerance of noise/hiss standards? I don't mind the latter if the trade off is that it gives me a more pleasing tone.
3) Also noticed the currently installed filter capacitor for the tremolo circuit (on the underside of the chassis) is labelled/rated at 32uf/350v rather than 450v. Should I worry and make the change to 450v or ok to leave as is?
4) The power tube cathode bias resistor appears to be two 220ohm resistors running in parallel, which I understand is a net 110ohm resistor. Noted original schematics show 50ohm, but perhaps that will cook the el84's too hot. I've got a Xircon 68ohm ceramic resistor I'm looking to replace them with, but again, anything I should check before sort of blindly making this switch? I do realize the tubes will run a bit hotter, but not to a point of danger to components, and I'm hoping to get a more pleasant tone. Again, any possible reason a previous tech overshot and went with 110ohm, which I think is pretty much on the extreme "cold" biasing side. And perhaps a very newbie question, I noticed one of the 4 el84's has become slightly microphonic (i.e. tapping with a chopstick results in a loud thump sound, but when playing or idle, i don't notice anything). I presume running a hotter bias will accelerate its death. I've read some debates about not needing to perfectly "match" the power tubes in the AC-30s because of the cathode biasing, would I be doing something extremely stupid in simply replacing the one power tube with another brand which will most likely be "unmatched"?
5) I've read post '67, Vox removed the cathode bypass cap in the early gain stage for the top boost channel. I just see a piece of wire between the two lugs on my unit so this appears to be consistent. Contemplating putting the 25uf capacitor back in as per original earlier top boost schematics. Is it just higher volume and earlier breakup or perhaps a different tone profile? Any idea why Vox did away with the capacitor in '67 in the first place?
6) And perhaps the most important item I'm trying to address. The amp currently sounds way too mid-rangey, even on the brilliant/top boost channel with treble maxed and bass/cut minimum. It sounds nothing like another mid-60's AC-30 I own. I'm only able to gain a semi-decent tone when adding some sort of treble booster in front, but ideally I'd like to avoid doing that as it still doesn't give me the crisp sparkly highs ("chime") I get from the other AC-30. Besides focusing on the below cap values (aim to replace any with 10-20% drift from "normal"), any other capacitors/resistors I should be measuring and focusing on to "open" up the tone so that it's less "nasal" and mid-rangey? On another thread, someone mentioned and suggested it may be the late 60's T1088's (albeit with original pulsonic cones) where Celestion may have slightly altered the cone/voice coils to make more efficient to compete in the high gain/wattage volume wars heading into the 70's. I've swapped out the original speakers with the other alnico silver bells from my mid-60's AC-30 and still getting an undesirable tone, so I'm focusing on a drift somewhere in the components. Any other "custom" changes/tweaks (from the original schematic values) most users have found to achieve more pleasant tones would also be appreciated.
That's all. Thanks again in advance for spending the time to read and provide any thoughts in pointing me to the right direction.
1) In attempts to make as period correct as possible, will look to change back all metal/carbon film resistors to carbon comp resistors. It appears the market supply of 1 watt carbon comps is severely depleted everywhere. May I confirm using 1/2 watts should be fine? I'm inclined to focus more on the tone signal path and perhaps leave metal film on the power side (and even the vib/trem circuit as I don't really use all that much)
2) I noticed the currently installed dual main filter capacitor "can" (C39, C40) is labelled/rated at 32uf/32uf rather than the expected 16uf/16uf as listed in the schematics. I'm tempted to switch this back, but I feel I should perhaps at least investigate possible reasons a previous tech may have installed the higher capacitance. I'm seeing mixed comments with regards to Vox possibly intentionally upping the value in the late 60's. Anything I should particularly measure or take a look at to avoid frying something by switching back to 16uf? Any difference in sound I should expect? Bear in mind this used to be modded and wired for solid state rectification, and I reverted back to GZ34 tube rectification. I see a lot of feedback that 16uf is way too small for "today's standards" but I'm not sure what that really means? Safety standards, or tolerance of noise/hiss standards? I don't mind the latter if the trade off is that it gives me a more pleasing tone.
3) Also noticed the currently installed filter capacitor for the tremolo circuit (on the underside of the chassis) is labelled/rated at 32uf/350v rather than 450v. Should I worry and make the change to 450v or ok to leave as is?
4) The power tube cathode bias resistor appears to be two 220ohm resistors running in parallel, which I understand is a net 110ohm resistor. Noted original schematics show 50ohm, but perhaps that will cook the el84's too hot. I've got a Xircon 68ohm ceramic resistor I'm looking to replace them with, but again, anything I should check before sort of blindly making this switch? I do realize the tubes will run a bit hotter, but not to a point of danger to components, and I'm hoping to get a more pleasant tone. Again, any possible reason a previous tech overshot and went with 110ohm, which I think is pretty much on the extreme "cold" biasing side. And perhaps a very newbie question, I noticed one of the 4 el84's has become slightly microphonic (i.e. tapping with a chopstick results in a loud thump sound, but when playing or idle, i don't notice anything). I presume running a hotter bias will accelerate its death. I've read some debates about not needing to perfectly "match" the power tubes in the AC-30s because of the cathode biasing, would I be doing something extremely stupid in simply replacing the one power tube with another brand which will most likely be "unmatched"?
5) I've read post '67, Vox removed the cathode bypass cap in the early gain stage for the top boost channel. I just see a piece of wire between the two lugs on my unit so this appears to be consistent. Contemplating putting the 25uf capacitor back in as per original earlier top boost schematics. Is it just higher volume and earlier breakup or perhaps a different tone profile? Any idea why Vox did away with the capacitor in '67 in the first place?
6) And perhaps the most important item I'm trying to address. The amp currently sounds way too mid-rangey, even on the brilliant/top boost channel with treble maxed and bass/cut minimum. It sounds nothing like another mid-60's AC-30 I own. I'm only able to gain a semi-decent tone when adding some sort of treble booster in front, but ideally I'd like to avoid doing that as it still doesn't give me the crisp sparkly highs ("chime") I get from the other AC-30. Besides focusing on the below cap values (aim to replace any with 10-20% drift from "normal"), any other capacitors/resistors I should be measuring and focusing on to "open" up the tone so that it's less "nasal" and mid-rangey? On another thread, someone mentioned and suggested it may be the late 60's T1088's (albeit with original pulsonic cones) where Celestion may have slightly altered the cone/voice coils to make more efficient to compete in the high gain/wattage volume wars heading into the 70's. I've swapped out the original speakers with the other alnico silver bells from my mid-60's AC-30 and still getting an undesirable tone, so I'm focusing on a drift somewhere in the components. Any other "custom" changes/tweaks (from the original schematic values) most users have found to achieve more pleasant tones would also be appreciated.
That's all. Thanks again in advance for spending the time to read and provide any thoughts in pointing me to the right direction.
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